This invention relates to a permanent magnet device, and more particularly to a magnetic field generator using a permanent magnet, which is suitable for generation of a homogeneous magnetic field. The permanent magnet device of the present invention finds its typical application in a MRI (magnetic resonance imaging) system.
A magnetic field generator using a permanent magnet generally comprises a member of a permanent magnet material and a magnetic path formed by a member of ferromagnetic material.
A typical prior art permanent magnet device designed for generating a homogeneous magnetic field has a structure as shown in FIGS. 1A and 1B. FIG. 1A is a front elevation view of the prior art permanent magnet device when viewed from the line IA--IA in FIG. 1B, and FIG. 1B is a side elevation view of the device when viewed from the line IB--IB in FIG. 1A. In the prior art permanent magnet device shown in FIGS. 1A and 1B, reference numerals 1, 1' and 2, 2' designate magnetic paths formed by members of a ferromagnetic material; 3 and 3' designate members of a permanent magnet material; 4 and 4' designate pole pieces; and 5 and 5' designate windows or holes which have the magnetic paths 2 and 2' respectively. A magnetic field Ho is generated in a gap space (a magnetic field generation space) 6 defined between the pole pieces 4 and 4'.
The homogeneity R (=.DELTA.Ho/Ho, where .DELTA.Ho represents an amount inhomogeneity of the magnetic field Ho at any point in the space 6) is given by EQU R.varies.10.sup.-(1+D/g) ( 1)
where D is the diameter of the pole pieces 4 and 4', and g is the length of the gap. Therefore, the larger the diameter D of the pole pieces 4 and 4', and the smaller the gap length g, the field homogeneity R is improved. When, on the other hand, the gap length g is fixed, the diameter D of the pole pieces 4 and 4' must be increased in order to improve the field homogeneity R. This means that the diameter of the permanent magnet members 3 and 3' must also be increased. Thus, when, for example, the diameter D of the pole pieces 4 and 4' is doubled, the required volume of the permanent magnet members 3 and 3' is about 2.sup.2 =4 times as much as that required when the pole piece diameter D is not increased. In the prior art device, therefore, ring shims (refer to Rose M. E. "Magnetic Field Corrections in the Cyclotron", Physical Review, 53, P715, 1938) or current shims (refer to Japanese Patent Publication No. 40-26368; Golay M. J. E.) have been provided on the both ends of the pole pieces 4 and 4', so that the ratio D/g can be minimized thereby improving the homogeneity R of the magnetic field Ho.
In FIG. 1A, all the amounts of the magnetic flux .phi. generated by the permanent magnet members 3 and 3' do not flow through the gap space 6 defined between the pole pieces 4 and 4'. Suppose, for example, that an Nd-Fe-B alloy, is used as the material of the permanent magnet members 3 and 3', the ratio D/g=2, and the strength of the magnetic field Ho=2 kG. In such a case, about 60% of the total flux .phi. returns directly to the magnetic paths 1, 1' and portions of the magnetic paths 2, 2' lying in the rear of the horizontal planes S - S' and S" - S"' of the respective pole pieces 4 and 4' and does not directly contribute to the generation of the magnetic field Ho in the gap space 6.
Between the horizontal planes S - S' and S" - S'", the amount of leakage magnetic flux between the magnetic paths 2 and 2' and the magnetic members 3 and 3' depends on the distance therebetween. When the distance between the permanent magnet members 3, 3' and the magnetic paths 2, 2' is short, the amount of leakage magnetic flux increases, and therefore the diameter of the magnetic paths 2 and 2' cannot be decreased. When the diameter of the magnetic paths 2 and 2' is about two times as large as that of the permanent magnet members 3 and 3', the leakage flux is about 15% of the total magnetic flux .phi.. The remaining amount, which is about 25% of the total magnetic flux .phi., flows through the gap space 6 between the pole pieces 4 and 4' and contributes to the generation of the magnetic field Ho.
As an example of known permanent magnet devices, that disclosed in JP-A No. 60-210804 is taken as a reference herein. This prior art discloses a permanent magnet device in which a magnetic field is generated in parallel to a gap space.
It will be apparent from the above description that, in the prior art permanent magnet device shown in FIGS. 1A and 1B, about 60% of the total amount of generated magnetic flux .phi. does not provide the inward pushing force acting upon the flow of magnetic flux through the gap space 6. Only about 15% of the total amount of magnetic flux .phi. provides the inward pushing force acting upon the flow of magnetic flux through the gap space 6, thereby homogeneizing the magnetic field Ho produced by the flow of magnetic flux through the gap space 6.
The prior art permanent magnet device shown in FIGS. 1A and 1B has the defect that the amount of magnetic flux acting to homogenize the magnetic field is small, and the ratio D/g must be increased in order to improve the homogeneity of the magnetic field generated in the gap space. Consequently, the prior art permanent magnet device is defective in that the size and weight of the magnetic field generator inevitably increase resulting in a high cost.